Water Utilization Characteristics Of Main Soil And Water Conservation Trees On Eastern Ordos Plateau

In semi-arid regions,water was a key limiting factor for plant growth.Soft rock area was located in the middle reaches of the Yellow River,due to the characteristics of the rock strata,it was highly susceptible to erosion,causing soil erosion to affect the middle and lower reaches of the Yellow River seriously.In order to understand the adaptative characteristics of the main tree species,three soil and water conservation trees,i.e.,Hippophae rhamnoides,Pinus tabuliformis and Armeniaca sibirica were chosen to study in soft rock area of eastern Ordos Plateau.Theδ¹⁸O value of xylem water of branch and potential water sources was measured,the contribution ratio of each potential water sources to different trees was calculated by multiple linear mixing model,the seasonal dynamic and inter-specific variation of water source was compared.Meanwile,theδ¹³C value of leaves was measured and their water use efficiency（WUE）was compared.In addition,the thermal dissipation probe technology was used to monitor the sap flow of the H.rhamnoides,and the soil moisture dynamics within 100 cm were simultaneously monitored to clarify the influence of rainfall and soil moisture on the sap flow of H.rhamnoides.The main findings were as follows:（1）All three tree species showed resource-dependent water use strategies.H.rhamnoides,P.tabuliformis and A.sibirica mainly used 0-10 cm soil water in May,accounting for 88.5%,94.0%and 91.6%of their total water resources.In July,H.rhamnoides mainly used 10-25 cm soil water（44.6%）and rain water（35.4%）;P.tabuliformis mainly used rain water（93.7%）;A.sibirica mainly used 25-100 cm soil water（55.9%）and rain water（36.8%）.In September,H.rhamnoides mainly used 25 cm and 75-100 cm soil water（88.9%）;P.tabuliformis mainly used0-10 cm and 50-75 cm soil water（84.5%）;whereas A.sibirica evenly used 0-100 cm soil water.（2）H.rhamnoides and P.tabuliformis could improve water use efficiency during drought.The WUE of H.rhamnoides in May was significantly higher than that in July and September.The WUE of P.tabuliformis in July was significantly higher than that in September.The WUE of H.rhamnoides was significantly higher than that of P.tabuliformis and A.sibirica in May and July.（3）Soil moisture and rainfall were important factors influencing the sap flow of H.rhamnoides.The sap flow had a time lag in response to rainfall and a significant correlation with the soil moisture content.At the beginning of the growing season,the soil moisture was at a low level,and during the middle and late growing season period,the soil moisture gradually increases due to rainfall infiltration.Rainfall determines the depth of soil that could be supplemented.The shallow soil moisture within 30 cm was supplied by rainfall event of 6.5 mm,however,the soil moisture witin 100 cm was supplied by rainfall event of 52 mm.The sap flow of H.rhamnoides was generally lower in the early growing season and higher in the middle and late growing season.Otherwise,the sap flow of H.rhamnoides recovered or exceeded the level before rainfall within1-4 days after rain.There was a significant positive correlation between the sap flow of H.rhamnoides and soil moisture of 0-100 cm,and soil moisture at 70-100 cm depth had a higher influence on the sap flow.In addition,compared with other months,the influence of soil moisture on sap flow of H.rhamnoides in June and July was stronger.Therefore,three trees chose to use different depths of soil water or rainwater based on their availability in different seasons.H.rhamnoides and P.tabuliformis could improve their WUE to adapt to environmental change during drought,which may be more suitable for local semi-arid environment than A.sibirica.There were seasnonal dynamic in soil moisture.Rainfall replenishes soil moisture,and the depth of infiltration depends on the rainfall.In addition to seasonal dynamics,the sap flow of H.rhamnoides was affected by rainfall and soil moisture and had a response time lag of 1-4 d.In June and July,soil moisture was an important factor limiting the sap flow of H.rhamnoides,and the soil moisture at 70-100 cm depth had a higher influence.